One of the main problems of the steam cracking process is the formation of a coke layer on the inner walls of the reactor coils. This leads on the one hand to a decrease of the heat flux to the reacting gas mixture and on the other hand to an increase of the pressure drop over the reactor. Hence, periodically the furnace operation has to be stopped and the coke has to be burnt off by means of a mixture of steam and air. To allow for simulation of the run length of industrial steam cracking coils the fundamental simulation model COILSIM1D 1,2,3 incorporates two coking models. The coking model of Plehiers 4-6 is developed for predicting coking rates for steam cracking of light hydrocarbon feedstocks. The model of Reyniers et al. 7 allows to simulate the coking rate of heavier feedstocks ranging from light naphtha fractions until heavy condensates. Both coking models account for the heterogeneous noncatalytic or so-called asymptotic coking only. The contributions of the heterogeneous catalytic coking and the homogeneous noncatalytic coking to the total amount of coke formed during the complete run length are assumed to be negligible. 4,5 The coking kinetics are coupled to the 1-dimensional reactor model equations solved in COILSIM1D.
These equations can be solved rapidly and accurately using a stiff solver DASSL8 resulting in the product yields and the pressure, concentration and temperature profiles, coking rates and coke layer thicknesses along the reactor coil for a specific reactor configuration. The boundary conditions can be either an external tube wall temperature profile, a process gas temperature profile or a heat flux profile. The latter can be obtained from Fluent9 or from one of the in house developed software codes FURNACE10 or FLOWSIM11. Moreover, the desired cracking severity can also be specified by means of e.g. the propylene/ethylene ratio, the methane/propylene ratio, a key component conversion, or a fixed yield of ethylene or methane. The program then returns the process conditions required to obtain the desired outlet specifications by solving the resulting two points boundary condition problem using a shooting method. Several examples of run length simulations will be presented for feedstocks ranging from ethane over n-butane to light and heavy naphtha's cracked over a broad range of operating conditions. Good agreement is obtained between simulated and observed run lengths of several industrial-cracking units. These simulations can be used in an industrial setting to optimize furnace operation for various feedstocks and operating conditions. To that purpose a graphical user interface has been developed. Its use will be demonstrated.
1. K.M. Van Geem, D. Hudebine, M-F. Reyniers, F. Wahl, J.J. Verstraete and G.B. Marin, Molecular reconstruction of naphtha steam cracking feedstocks based on commercial indices. Comp. & Chem. Eng., 31 (9): 1020-1034, 2007
2. K.M. Van Geem, M-F. Reyniers, and G.B. Marin, Challenges of Modeling Steam Cracking of Heavy Feedstocks, Oil & Gas Science and Technology-Revue de l'IFP, 63, 79-94, 2008.
3. K.M. Van Geem, M-F. Reyniers, and G.B. Marin, Taking optimal advantage of feedstock flexibility with Coilsim1D, AICHE Spring meeting, New Orleans, LA, 2008.
4. P.M. Plehiers, G.C. Reyniers and G.F. Froment, “Simulation of the Run Length of an Ethane Cracking Furnace”, Ind. Eng. Chem. Res., 29, 636-644, 1990.
5. P.M. Plehiers and G.F. Froment, Chemicla engineering Communications, 80, 81-99, 1989.
6. M.V.R. Rao, P.M. Plehiers and G.F. Froment, Chemical Engineering Science, 43, 6, 1223-1229, 1988.
7. G.C. Reyniers, Froment G.F., F.D. Kopinke and G. Zimmerman, Coke formation in thermal cracking of hydrocarbons. 4. Modeling coke formation in, naphtha cracking, Ind. Eng. Chem. Res. 33, 2854-2850, 1994
8. S. Li, L.R. Petzold, Design of New DASPK for Sensitivity Analysis, UCSB Technical report, 1999.
9. G.D. Stefanidis, K.M. Van Geem, G.J. Heynderickx and G.B. Marin, Evaluation of high-emissivity coatings in steam cracking furnaces using a non-grey gas radiation model, Chemical Engineering Journal, 137 (2), 411-421, 2008.
10. G.J. Heynderickx, M. Nozawa, Banded gas and nongray surface radiation models for high-emissivity coatings. AICHE JOURNAL 51 (10): 2721-2736 OCT 2005
11. G.D. Stefanidis, B. Merci, G.J. Heynderickx and G.B. Marin, CFD simulations of steam cracking furnaces using detailed combustion mechanisms. Comp. & Chem. Eng. 30 (4): 635-649, 2006.